1. Field of the Invention:
The present invention relates to decontamination of lightly radio-contaminated metals, and in particular to decontamination of radio-contaminated metals by reductive electrochemical processing. Of particular interest to the present invention is the remediation of radio-contaminated nickel from decommissioning the uranium gaseous diffusion cascades from the Department of Energy's K-25 Plant in which nickel is the primary constituent. Spectrochemical analyses of spot samples of the K-25 nickel indicate that the nickel is about 99.8% pure and contains small amounts of copper, iron, chromium, magnesium, silicon, cobalt, titanium, aluminum and carbon. Radiochemical analyses indicate that the K-25 nickel is contaminated with about 0.5 parts per billion (ppb) neptunium 237 or less, about 0.005 ppb plutonium 239 or less, about 0.85 parts per million (ppm) technetium, about 0.93 ppm uranium 235 and about 1.74 ppm total uranium. Also, this nickel has been determined to have gross beta activities of about 4000 Bq/gm. Therefore, it cannot be freely recycled because the international release criteria for permitting the unrestricted circulation of nickel free from governmental health control is not greater than 74 Bq/gm. The release criteria in particular countries may be as low as 0.37 Bq/gm. The decontamination art taught herein applies equally well to the recovery and decontamination of other multivalent, strategic metals which can be electrowon such as copper, cobalt, chromium, iron, zinc, aluminum and like transition metals.
2. The Prior Art:
The radiochemical decontamination art is presented with unique practical problems not shared with traditional extraction technologies. Radiochemical extraction technologies are generally concerned with the economic recovery of "product radiochemicals". Routine process inefficiencies which permit residual (ppm or less) amounts of radiochemicals to remain in process streams or in byproducts raise only normal economic issues of process yield and acceptable process costs. Such inefficiencies render these processes unacceptable where, as here, the nickel purity with respect to radionuclides must exceed 99.9999999% (or so called "nine - 9s").
Various decontamination processes are known in the art, and specifically for decontamination of nickel. Nickel can be removed by selectively stripping from an acidic solution by electrowinning. See U.S. Pat. No. 3,853,725. Nickel may also be removed by liquid-liquid extraction or solvent extraction. See U.S. Pat. Nos. 4,162,296 and 4,196,076. Further, various phosphate type compounds have been used in the removal of nickel. See U.S. Pat. Nos. 4,162,296; 4,624,703; 4,718,996; 4,528,165 and 4,808,034. In addition, electroslag processing has been proposed, but the extent of decontamination can not be verified in the course of the processing because nickel is self-shielding so that highly contaminated product must be recycled back to the beginning of the process.
It is known that metallic nickel, contaminated with fission products, can be decontaminated to remove any actinides present by direct electrochemical processing based on the differences in reduction potential in the electromotive force (emf) series. Actinide removal is favored by two phenomena during electrochemical plating. Actinides have a significantly higher reduction potential relative to nickel and they are normally won from molten salt electrolyte rather than from aqueous electrolyte. See U.S. Pat. Nos. 3,928,153 and 3,891,741, for example. Other electrolytic processes are disclosed by U.S. Pat. Nos. 3,915,828; 4,011,151; 4,146,438; 4,401,532; 4,481,089; 4,537,666; 4,615,776 and 4,792,385.
While the removal of uranium and other actinides has been generally addressed by electrorefining, the removal of technetium has continued to be a substantial problem. A major stumbling block in developing process technology for decontaminating homogeneously contaminated metals to less than about 74 Bq/gm (for recycling free of governmental controls) has been the lack of effective in-process decontamination verification and reprocessing of technetium-contaminated process streams.